High purity olefins, such as propylene and butadiene, have traditionally been produced through the process of steam and/or catalytic cracking. For example, propylene is usually produced as one of the main by-products in an ethylene plant or as a secondary by-product in a refinery utilizing a fluid catalytic cracker. Because of the limited efficiency of existing industrial systems and the high cost of petroleum sources, the cost of producing high purity olefins has been steadily increasing.
Polymer-grade propylene is required for the production of polypropylene and useful for the production of other propylene derivatives. Polymer-grade propylene is characterized by very low concentrations of impurities, including low levels of paraffins (saturated hydrocarbons) such as propane, ethane, and butane. Commercial chemical-grade propylene, unlike polymer-grade propylene, is characterized by higher concentrations of saturated hydrocarbons. Typical chemical-grade propylene purities range from 95% to 99.0% propylene, and for polymer grade propylene the purity is typically above 99.5%.
Another olefin that is often produced in commercial chemical plants is butadiene. Butadiene is a versatile raw material used in the production of a wide variety of synthetic rubbers, polymer resins and chemical intermediates. The largest uses for butadiene are the production of styrene butadiene rubber and polybutadiene rubber, which are used mainly in tire products. Butadiene is also one of the components used in the manufacture of acrylonitrile-butadiene-styrene, styrene-butadiene copolymer latex, styrene-butadiene block copolymers and nitrile rubbers.
Processes for direct propylene production include technologies directed specifically to conversion of C4 hydrocarbons to propylene, such as Olefins Conversion Technology (OCT) from Lummus Technology, Inc. and the CATOFIN® dehydrogenation process. In the OCT process, n-butenes from C4 feed are reacted with ethylene to produce polymer-grade propylene in a fixed bed catalytic metathesis reactor. The catalyst promotes two primary chemical reactions in the OCT process: (1) propylene is formed by metathesis of ethylene and 2-butene; and (2) 1-butene is isomerized to 2-butene as 2-butene is consumed in the metathesis reaction. The CATOFIN® dehydrogenation process uses a fixed-bed reactor having a catalyst selected to optimize conversion of propane to propylene.
Other methods of propylene production have been described. For example, U.S. Pat. No. 6,420,619 describes the production of propylene using successive distillation, hydrogenation and isomerization of a C3-C6 hydrocarbon cut from a cracking process to form 2-butene. Thereafter, the 2-butene is catalytically metathesized with ethylene to form propylene. U.S. Pat. No. 7,074,976 describes the production of propylene from olefins using a combination of hydrogenation, isomerization and disproportionation to form internal linear olefins. Thereafter, the internal linear olefins are converted to propylene.
Butadiene can be produced using the CATADIENE® process. This technology is a single step process for catalytic dehydrogenation of light hydrocarbons to produce diolefins of the same carbon number. Using the CATADIENE® process C4 feedstocks may be converted to butadiene. Other butadiene production processes have been described. For example, U.S. Pat. No. 7,417,173 describes the production of butadiene from n-butane using dehydrogenation, condensation and phase separation to produce a product stream consisting substantially of butadiene.
None of the processes for production of propylene or butadiene discussed above describe the conversion of low value feedstocks comprising butane to produce both propylene and butadiene. There exists an ongoing and unmet need in the industry for improved, economical and efficient methods for producing olefins, such as propylene and butadiene, in industrial chemical processes. The present invention overcomes the deficiencies of the prior art by producing both propylene and butadiene in an integrated process.